This invention relates to means for producing pressure within an assembly to equalize with the ambient pressure around the assembly and is particularly useful for assemblies intended for use in a deep ocean environment.
Devices and instrumentation used in a deep ocean environment are subjected to large hydrostatic pressures on the order of 0.5 psi per foot of depth, or a maximum of about 10,000 psi at a depth of 20,000 feet. In order to prevent the pressure from damaging the devices, the devices are generally housed in some type of pressure vessel, typically constructed of steel strong enough to withstand the large hydrostatic pressures. Such pressure vessels are typically quite bulky and heavy.
Although some devices are damaged directly by pressure and thus must be protected by a suitable pressure vessel, other devices are not damaged directly by pressure but rather by pressure-induced forces. For example, a normally-sealed device such as a transistor may be rendered inoperable if its can is forceably ruptured. However, if the can were opened in such a way that external pressure, but no contaminents, were to enter the normally sealed area, then regular operation of the device would continue. This is sometimes accomplished by means of an inert fluid which serves to transmit the hydrostatic pressure to the components while protecting them from direct contact with the ocean. The inert fluid conveys the hydrostatic pressure outside the vessel to the operating components but is prevented by a flexible diaphragm from mixing with the ocean water. A disadvantage of using a fluid to transmit pressure is that the viscosity of the fluid can prevent proper operation of the device, especially if the operation involves mechanical motion.
Semi-rigid substances such as epoxies or potting compounds have also been employed to transmit the hydrostatic pressure. Even more so than fluids, however, semi-solids may cause flexure and mechanical failure of the embedded components when the hydrostatic pressure increases.
These problems can be largely eliminated if a gas is used for pressure equalization rather than a fluid or semi-rigid substance. Heretofore, devices utilized in pressure equalization have stored such gas beforehand inside the submerged vessel itself. This has proved unsatisfactory because of the difficulty of obtaining and storing the gas at the required maximum pressures and volumes. For example, to protect a one-liter volume at 8000 psi (545 atmospheres) would require 545 liters of gas at one atmosphere pressure. The gas would have to be stored in the underwater vessel at a pressure in excess of 8000 psi. Indeed, to effect a net volume saving, the gas has to be stored at an even higher pressure--on the order of five times 8000 psi or 40,000 psi. Therefore, a trade-off exists between the overall vessel dimensions and weight versus the quantity of gas enclosed.